Exposure device for manufacturing color picture tubes

ABSTRACT

An exposure device for projecting aperture patterns from a mask onto a photosensitive layer on a picture screen for cathode-ray tube comprising a conical light source and a substantially rotationally symmetrical negative lens between the light source and the mask to diverge the light thereby enlarging the transverse dimensions of the light source as viewed from the mask. The dimensions of the light source are increased to compensate for any loss in brightness from the lens.

United States Patent Inventors Appl. No. Filed Patented Assignee Priority Piet Gerard Joseph Barten;

Johannes Cornelis Adrianus Vannes, both of Emmasingel, Eindhoven, Netherlands 881,464

Dec. 2, 1969 Dec. 21, 1971 U.S. Philips Corporation New York, N.Y.

Dec. 4, 1968 Netherlands EXPOSURE DEVICE FOR MANUFACTURING [56] References Cited UNITED STATES PATENTS 2,936,682 5/1960 Krawitz 2,942,099 6/1960 Goldstein 3,420,150 1/1969 Kaplan Primary Examiner-John M. Horan Assistant Examiner-Thomas J. Mauro Attorney-F rank R. Trifari ABSTRACT: An exposure device for projecting aperture patterns from a mask onto a photosensitive layer on a picture screen for cathode-ray tube comprising a conical light source -COLOR PICTURE TUBES and a substantially rotationally symmetrical negative lens 4Claims,4Drawing Figs. between the light source and the mask to diverge the light thereby enlarging the transverse dimensions of the light source U.S. Cl 95/1 as viewed from the mask. The dimensions of the light source G03b are increased to compensate for any loss in brightness from 95/l the lens.

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INVENTORS PIET G. J. BARTEN JOHANNES C.A.VAN ES BY r PATENTED nc21 I97! 316283129 SHEET 2 BF 2 INVENTOR- PIET G. J. BARTEN JOHANNES C.A. VA N NES BY h AGE/V7 EXPOSURE DEVICE FOR MANUFACTURING COLOR PICTURE TUBES The invention relates to an exposure device for manufacturing image screens for cathode-ray tubes for displaying colored images, which device is provided with a light source by which a pattern of apertures in a mask is optically projected onto a photosensitive binder provided on a support, the path of light from the light source to the support passing through a substantially rotation-symmetrical negative lens, the light intensity 13 from the light source to a point on the edge of the support satisfying the condition:

wherein: B is the angle between the line of connection between the center C, of the light source and the center C of the support and the line of connection between C, and the point on the edge of the support; and la is the light intensity of the light source at an angle a with C,C,,, wherein a is given by the relation: 7

sin a=(1+v/f) sin B wherein: v is the distance in mm. from the center C, to the lens and f is the focal distance in mm. of the lens.

The said support is generally a transparent cover glass which forms part of the future picture screen for the said cathode-ray tube. A support (and a layer possibly provided thereon) of a future picture screen is sometimes also called face plate.

The cathode-ray tube mentioned in the preamble is, for example, a color television display tube; it may be, however, alternatively a tube in which colors are made visible on the screen for other purposes.

The light source is, for example, a primary light source having a given light distribution such as a lamp having an internal reflector, or it is a secondary light source. An example of the latter type is weakly matted disc irradiated by a projector. A further example of the latter type is a tip of a conelike light conductor, whereby the lower side of the cone receives light.

A cathode-ray tube for displaying colored images generally includes a system of electrodes which supplies a few electron beams the electrons of which move along different paths through the apertures in a mask towards the screen for generating the so-called primary colors. These are generally the colors green, red and blue. To this end a very large number of dots (or stripes) is generally provided in the screen a number of which comprises a luminescent substance which radiates light of one of theprimary colors (for example, green) upon electron incidence; other dots on the screen comprise a luminescent substance which radiates light of one of the other primary colors (for example, red) upon electron incidence. A third group of dots may comprise, for example, a blue luminescent substance. However, it is not necessary for the number of different dots and the number of electron beams to be three.

The said screen generally includes a collection of a very large number of groups of dots. One group of dots consists of a combination of dots of different composition.

In a known device each electron beam covers (through the mask) only the dots of one and the same composition. By varying the ratio between the intensities of the electron beams different colors can be made visible on the screen.

It is known to provide the said dots on the screen by means of a process in which a layer provided on the faceplate and comprising a luminescent substance and a photoresist is exposed through the mask (color selection mask) by light rays which impinge upon the screen at areas where later the electrons of one of the electron beams will impinge upon the screen. Due to the exposure the luminescent substance is then adhered to the screen at these areas. The substance at the unexposed areas of the screen may be rinsed off. The dots remain. In a following manufacturing stage a layer comprising a different luminescent substance but likewise provided with a photoresist is provided on the faceplate. This new layer is now exposed at such areas that the new dots" will come at the areas to be impinged upon later by the second electron beam. The latter dots thus are not located at the same areas as the previously provided dots. If necessary this process may be repeated for a third layer, etc. The exposure" is always effected by means of radiation to which the photosensitive binder reacts. This may be, for example, visible radiation or, for example, ultraviolet light. It is alternatively possible to expose a layer which comprises the photosensitive binder only and to provide the luminescent substance only at a later stage.

A known device of the kind described in the preamble is shown, for example, in FIG. 7 of US. Pat. specification No. 2,936,682. It is true that the said patent specification gives little information about the light distribution of the tip of the cone 83 but it seems very acceptable that in that case the light intensity will have a greatly decreasing variation in a given direction plotted as a function of the angle which is made by this direction with the line of connection of the tip of the cone and the center of the support 73. This variation will very probably have a greater decrease than is shown by the formula:

Iy=10cos wherein I7 is the light intensity in a direction which forms an angle 7 with the above-mentioned line of connection and I0 is the light intensity in the direction which substantially coincides with the direction of the mentioned line of connection.

If two values for ynamely 'y, and 7,, wherein 7,, are substituted in the formula I'y I0 cos 'y, then it can be derived that Iy, cos'y,,/cos 7,-ly which is comparable with lfi cos B/cos y-Ia so that such a light distribution satisfies the condition mentioned in the preamble.

A different formulation of this condition is that the brightness of a surface element of the light source (that is to say, of an element of the tip of the cone in the case of the patent specification referred to) will be smaller for larger angles with the vertical.

A drawback of the mentioned known device is that additional auxiliary means are usually necessary to maintain the ratio between the illumination value at the area of the center of the support (the screen) and at the area of the edge of the support below an acceptable limit. In general much more light must be radiated towards a spot on the edge of the support than to the center of the support because this edge is farther remote from the light source than the center and furthermore because the apertures of the mask at the area of the edge are more often smaller than those at the center of the support.

A known auxiliary means for obtaining a better illumination distribution on the screen is the use of a filter layer in the light path from the light source to the screen, the permeability of the filter layer being smaller for rays towards the center of the screen than for rays towards the edges. Such a filter causes loss of light which is at the expense of the exposure time.

A further proposed auxiliary means for enhancing the light intensity distribution on the support consists in using a slightly tilted lens between the light source and the support. This, however, has the drawback of an asymmetry in the illumination distribution on the screen.

It is an object of the present invention to obviate or at least mitigate these drawbacks.

An exposure device according to the invention for manufacturing image screens for cathode-ray tubes for displaying colored images, which device is provided with a light source by which a pattern of apertures in a mask is optically projected onto a photosensitive binder provided on a support, and in which the light path from the light source to the support passes through a substantially rotation-symmetrical negative lens and in which the light intensity 113 from the light source to a point on the edge of the support satisfies the condition:

I[3 cos B/cos a-Ia wherein: B is the angle between the line of connection between the center C, of the light source and the center C of the support and the line of connection between C, and the point on the edge of the support; and la is the light intensity of the light source at an angle of a with C,C,, wherein a is given by the relation:

sin oz=( l+v/f"-sin B wherein: v is the distance mm. from the center C, to the lens; and f is the focal distance in mm. of the lens, is characterized in that the lens satisfies the condition:

An advantage of this exposure device is that due to the comparatively great divergence of the beam of light from the light source the possibility is created to enlarge the transverse dimensions of the light source. It is true that the spread of the beam of light, which in itself results in an improvement of the center-toedge ratio of the illumination on the screen, sometimes goes at the expense of the overall quantity of light which is projected onto the screen. However, by simultaneously choosing the transverse dimensions of the light source to be larger, and to at least maintain the brightness thereof, the quantity of light radiated by that source to the screen is increased. This has a compensating effect on the sometimes partially detrimental influence of the above-mentioned divergence. In addition the larger transverse dimension of the light source compensates the reduction of the image from that source which image is obtained with the negative lens. The maximum admissible transverse dimension of the light source is detennined inter alia by the properties of the photosensitive layer, the distance between the. light source and the screen, the size of the apertures of the mask, etc.

The negative lens in the embodiment of this U.S. patent specification referred to is arranged in such a position and has such a refraction that the reduction of the tip of the cone as viewed from the screen is negligibly small. This lens also serves for completely different purposes.

The reduction factor is smaller than three-fourths in the case of the present invention. This may be explained as follows:

According to the invention there should apply that f 3v. The use of formula M=( l/ l+(v/f)) wherein M is the reduction, yields for f=3v: M=%. For f 3v this formula yields M The theoretical background of the invention may be explained as follows:

A prior situation 1 (prior to the invention) and a novel situation ll (subsequent to the invention) are considered.

In situation I a cone tip having a transverse dimension d was used which without the interposition of a (strong) negative lens projected light onto a support provided with a photosensitive binder.

ln situation ll a cone tip having a transverse dimension D is used which projects light through a negative lens onto a support which is provided with a photosensitive binder.

Apart from a correction factor g the light distribution of the tip of the cone of dimension D is assumed to be the same as that of the tip of the cone of dimension d. Furthermore it is assumed that D d and D/d).

ln situation I the light intensity of the tip of the cone into the direction of the center C of the support is given, for example by 1,, and is given by l, into the direction of the edge of the support.

In situation ll the light intensities into these directions (after refraction by the negative lens) are referred to as 1,, and l,,,,, respectively.

The light intensity into a certain direction will now become g times larger due to the use of the larger tip of the cone. However, by interposition of the negative lens each part of the beam of light is deflected and is furthermore'diverged over a larger solid angle. lf 8, is the original angle of radiation and 8,, is the angle of radiation after deflection, it can be derived that:

sin 8,=( l+v/f)*' sin 6,, wherein v and f are the object and focal distances, respectively, of the lens.

The divergence in the larger solid angle obtained in situation ll implies that the light intensity after refraction by the lens is reduced by a given factor. This factor is:

If this is combined with a cone tip which is 3 times larger, a variation of light intensity in situation II as compared with situation I by a factor of k=g-( l+v/f) cos 8,,lcos 6, is obtained.

If viewed from the center C, of the support the apparent tip of the cone in situation I is equal to that in situation ll, then 3 should be equal to l+v/f)". in that case the factor For a ray from the light source to the center C, of the support: S -S i), thus the factor k is in this case equal to one. In other words in the direction C l,=l,,.

For a point of the support the light intensity is I, and l,,,,, respectively. The angle of radiation 8, is in this case a and 8,,== B. The factor k thus is cos fi/cos 01. Hence: I,,,,=cos fi/cos a-Ia.

in a device according to the invention lfl cos B/cos a-la. Hence IB I,,,,. Or, in other words, the new light intensity I,,,, into the direction of a point in the edge of the support is larger than the prior light intensity l,,,(=lfl) to that point on the edge of the support. The light intensities to the center of the support were equal in the situations I and ll as was derived above. Hence situation ll (that is to say, the situation subsequent to the present invention) with the greater light intensity to the edge of the support provides an advantage relative to situation I. In fact, in this case II the light absorption (in a transmission filter) used for obtaining the correct illumination-distribution on the support can be reduced on its path from C l to C For manufacturing image screens in which the photosensitive binder consists of a photoresist which is soluble in water and which contains a bichromate salt as a sensitizer the dimensions of the light source in mm. transverse to the connection from C to C, are preferably at least equal to:

0.7-L-B/Q, wherein B is the dimension in mm. of an aperture in the central part of the mask; and

L is the distance in mm. between C and C and Q is the distance in mm. between the central part of the mask and the central part (C of the support.

An advantage of this preferred solution is that a possible loss of light due to the divergence of the beam is compensated for at least for the greater part by the choice of comparatively large transverse dimensions of the light source.

With regard to these transverse dimensions for the light source the following can be stated with reference to FIG. 1. In this FIG. 1 the reference C is the diameter of a luminous surface element which is provided at a distance P from a mask M, the distance between mask M and a screen S being denoted by 0. An aperture in the mask is circular and has a diameter B. The diameter between the limit of the half-shadow at S and the unexposed parts is indicated by D max. The cone bounded by the circumference of the aperture of the mask of diameter B and the center of C as atip terminates on S in a circle of diameter Do.

Now it can be derived that D max (LB+CQ)/P and D0=LBIP This yields: D max/Do=l+CQ/LB.

Acceptable values for D max/D0 are D max/D0 s 1.6. Hence in the case without a negative lens there should apply that CQ/LB s 0.6, that is to say, Cs 0.6 LB/Q.

When using the negative lens according to the invention the image of C is replaced by C, wherein C' s 36 C.

Thus in that case Y4 C s 0.6LB/Q or C s 0.8 LB/Q.

In this preferred case the inventors draw the lower limit at C=0.7LB/Q by which is meant that a light source is used having larger transverse dimensions than is common practice in those cases in which no lens according to the invention is provided.

The lens of an exposure device according to the invention has preferably a concave boundary surface facing the light source wherein the center of curvature of the boundary surface lies on the line of connection between the light source and the center of the lens and wherein the second boundary surface of the lens is part of a convex surface, the center of this convex surface being located on a straight line through the light source and the center of the lens, the second boundary surface having a convex shape and the conditions:

R,=v/( l+n) and R =(l +(l/n) R,+t being substantially satisfied wherein: R is the radius of curvature in mm. of the first boundary surface; and

v is the distance in mm. between the center C and the lens; and

n is the relative refractive index of the material of the lens for the wavelength range of the light from the light source; and

R is the radius of curvature in mm. of the second boundary surface; and

t is the thickness in mm. of the central part of the lens.

If both conditions are satisfied the optical system is aplanatic. A light ray then impinges upon the second boundary surface of the lens at a straight angle. An advantage of the aplanatic system is that at least one point of the light source is projected without error.

In order that the invention may be readily carried into effect an embodiment thereof will now be described in detail by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1 shows a figure already described and relating to the course of the light rays from a luminous surface element having a diameter C, 7

FIG. 2 is a diagrammatical cross section of an exposure device according to the invention;

FIG. 3 shows a few details of the exposure device of FIG. 2 at an enlarged scale;

FIG. .4 shows a light distribution of a tip of a light conductor of the exposure device of FIG. 2.

In FIG. 2 the reference numeral 1 denotes a substantially closed container of approximately 1 m. height. The reference numeral 2 denotes a cover for this container. This cover 2 is provided with an aperture 3. Three or more stops 4 (one of which is visible in FIG. 2) and a positioning notch 5 are provided near the aperture 3. A screen 6 is urged against the stops 4 with the aid of a notch 5. This serves to have the screen 6 occupy a given position. The screen 6 is a future 25-inch image screen of a color television display tube. A mask 7 which is provided with a plurality of holes 8, 8 etc. is provided at a short distance from this screen. The container 1 contains a secondary light source 9 which is formed by the tip of a conelike light conductor 10. The source 9 radiates light via a negative lens 11 (according to the invention) and via a correction lens 12 through the apertures 8, 8 etc. of the mask 7 onto the screen 6. A photosensitive layer provided on the side of screen 6 facing the cone 9 is then exposed. This layer consists of a photoresist which is soluble in water and contains a bichromate salt as a sensitizer. The correction lens 12 serves for adaptation of the direction of the light rays to the direction of the deflected electron beam (in the tube to be manufactured). A filter layer 12a whose permeability increases from its center towards the edge is provided on the lens 12. The reference numerals l3 and 14 denote two electric wires which serve to supply an electric discharge lamp which projects light onto the base plane of the cone 10. This lamp and the cone are mounted in an auxiliary member 15 which is supported by a few vertical consoles l6 and 17. The distance between the center C of the tip of the cone and the lens 11 was 25 mm. in a certain embodiment. The distance between C and a point C, which represents the center of screen 6, was then 352 mm. An aperture of the mask near the point C had a diameter of approximately 0.29 mm. The central portion of the lens 11 had a thickness of approximately 3 mm. The tip 9 of the cone had a diameter which was approximately 1.5 times larger than is common practice, namely 6 mm.

In FIG. 3 the lens 11 and the tip 9 of the cone of FIG. 2 are shown at an enlarged scale. In this Figure the reference I-l represents the centerline through the center of the tip C of the cone and the center of the lens 11. The reference 0 is a point of intersection of this centerline H with the lens 11. The reference K denotes the center of curvature of the first boundary surface and K, denotes the center of curvature of the second boundary surface of the lens 11. The reference C JG denotes the course of a ray of light from the tip of the cone through the lens 11 to the screen 6. This ray of light intersects the second boundary surface at a straight angle.

The radius of curvature of the first boundary surface was 10 mm. in the relevant case and it was 19.8 mm. for the second boundary surface. The refractive index n of the lens was 1.48 for the light from the lamp.

It can be calculated from these data that, viewed from the center C, (see FIG. 2), the reduction M of the tip 9 of the cone by the lens 11 is approximately 0.68, that is to say, it is less than three-fourths so that f 3v is satisfied. Furthermore, the size of the tip of the cone (6 mm.) is larger than 0.7-LB/Q=5.4 wherein L is the distance between the tip of the cone and the point C, (352 mm.) and B is the diameter of the aperture of the mask (diameter 0.29 mm.) at C and Q is the distance between mask and screen (13,5 mm.).

In FIG. 4 the reference C denotes the central point of the previously mentioned cone 10. The reference I, in this Figure denotes the light intensity into the direction of the point C,. The reference TB denotes the light intensity of the tip of the cone into the direction of a point on the edge of the support. This direction constitutes in the embodiment an angle fl=45 with the axis C C through the point C,. The magnitude of the light intensity into other directions is denoted by the curve 21 in FIG. 4. This curve shows that the light intensity is smaller at a greater angle with C C By interposition of the lens 11 (see FIGS. 2 and 3) the ratio between the light intensities towards C and into the direction [3 is improved, however. This is apparent, for example, from FIG. 3. In fact, when using the negative lens the light intensity into the direction B is generated by a ray of light which is radiated by the cone 10 at an angle a, that is to say, at a smaller angle. The quantity of light actually radiated by the cone 10 in combination with the lens 11 into the direction [3 is denoted by I in FIG. 4. This light intensity is l, =cos Blcos a-Ia. This light intensity I is greater than the light intensity IB. In the relevant embodiment a was approximately 2840.

The result of the introduction of the negative lens is therefore that the filter layer which is generally applied on the lens 12 (see FIG. 2) may be less thick in the center of this lens. In the case of FIG. 2 referred to the light absorption in the center of the lens 12 was only approximately 2.5 times that of the edge of the lens 12. As a result less loss of light occurs in the filter layer. In combination with the cone 9 having a tip diameter which is approximately 1.5 times larger than is common practice this leads to shorter exposure times of the screens 6.

Since the lens is an aplanatic lens as is apparent from the mentioned radii of curvature and distances, at least the point C, is displayed on the screen 6 without error. This is advantageous for obtaining phosphor dots of the desired shape on the screen.

What is claimed is:

1. An exposure device for projecting aperture patterns from a substantially planar mask onto a photosensitive layer on a picture screen for cathode-ray tubes comprising a conical light source having a longitudinal axis orthogonal to the plane of said mask, and a substantially rotationally symmetrical negative lens between said light source and said mask, the focal length f in mm. of said lens satisfying the condition:

f 3v wherein v is the distance in mm. between the center of the light source and the lens, said lens diverging the light radiated from said light source to enlarge the transverse dimensions of said light source as viewed from said mask, and wherein the light intensity I, from said light source to a point on the edge of said mask satisfies the condition:

I cos B/cos a-I 0 wherein B is the angle between a line connecting the centers of said light source and said mask and the edge of said mask, I a is the light intensity at angle a with said line connecting the centers of said light source and said mask, and a is given by the relation sin oz=( 1+v/f) ".sin B, said light source having dimensions which substantially compensate for any loss in brightness resulting from the divergence of he light radiated from said source.

2. An exposure device as claimed in claim 1 wherein said photosensitive layer comprises a photoresist that is soluble in IOIOIS 0079 water and contains a bichromate salt as a sensitizer, and the dimensions of said light source in mm. are at least equal to 0.7-LB/Q wherein: B is the diameter in mm. of an aperture in the central part of said mask, L is the distance in mm. between the centers of said light source and said mask, and Q is the distance in mm. between the centers of said mask and said screen.

3. An exposure device as claimed in claim 1 wherein said negative lens comprises a concave boundary surface facing said light source having a center of curvature lying on a line connecting the centers of said light source and said lens, and a convex boundary surface opposite said light source having a center located on a straight line through said light source and center of said lens, said convex boundary surface substantially satisfying the condition R =v/(l+v) and R =(l+l/n) R +t wherein: R is the radius of curvature in mm. of the concave boundary surface, v is the relative refraction index of the material of the lens for the wavelength range of the light from the light source R, is the radius of curvature in mm. of the convex boundary surface and t is the thickness in mm. of the central part of the lens.

4. An exposure device as claimed in claim 1 further comprising a corrective lens between said lens and mask for adapting the direction of said light radiated from said light source to the direction of deflected electron beams, said corrective lens having a filter layer, the permeability of said filter layer increasing from its center towards its edges.

IOIOH MRO gg gg UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION f Patent 3,628,429 Dated Hangman-.2] 191]:

Inventor($) I'PIET G. J, BARTEN ND It is certified that error appears in the ab ve-identified patent and that said Letters Patent are hereby corrected as shown below:

r In the title page, change c'o-inventor's name from "Johannes Corneli s Adrianus vannes" to -Johannes Cornel-is Adrianus Van Ne s-.

Signed and sealed thisi h T day f M 1972.

(SEAL) Attesit: v EDWARD M.FLETCHER, JR. ROBERT GO'TISCHALK Attesting Officer Commissioner of Patents 732 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,628,429 Dated December; 2 191 Inventor(S) PIE'I' G. J, BARTEN AND JOHANNES c. A. VAN mas It is certified that error appears in the above-identified patent and that: said Letters Patent are hereby corrected as shown below:

In the title page, change c'o-inventor's name from Johannes Cornelis Adrianus Vannes" to -Johannes Corhel'is Adrianus Van Nes--.

Signed and sealed thisJ -h K day 1 Ma 1972.

I (SEAL) Attest:

EDWARD M.FLETCHER, IR. ROBERT GO'I'TSGHALK Attesting Officer Commissioner of Patents 

1. An exposure device for projecting aperture patterns from a substantially planar mask onto a photosensitive layer on a picture screen for cathode-ray tubes comprising a conical light source having a longitudinal axis orthogonal to the plane of said mask, and a substantially rotationally symmetrical negative lens between said light source and said mask, the focal length f in mm. of said lens satisfying the condition: f<3v wherein v is the distance in mm. between the center of the light source and the lens, said lens diverging the light radiated from said light source to enlarge the transverse dimensions of said light source as viewed from said mask, and wherein the light intensity IB from said light source to a point on the edge of said mask satisfies the condition: IB<cos B/cos .I Alpha wherein B is the angle between a line connecting the centers of said light source and said mask and the edge of said mask, I Alpha is the light intensity at angle Alpha with said line connecting the centers of said light source and said mask, and Alpha is given by the relation sin Alpha (1+v/f) 1.sin B, said light source having dimensions which substantially compensate for any loss in brightness resulting from the divergence of he light radiated from said source.
 2. An exposure device as claimed in claim 1 wherein said photosensitive layer comprises a photoresist that is soluble in water and contains a bichromate salt as a sensitizer, and the dimensions of said light source in mm. are at least equal to 0.7.LB/Q wherein: B is the diameter in mm. of an aperture in the central part of said mask, L is the distance in mm. between the centers of said light source and said mask, and Q is the distance in mm. between the centers of said mask and said screen.
 3. An exposure device as claimed in claim 1 wherein said negative lens comprises a concave boundary surface facing said light source having a center of curvature lying on a line connecting the centers of said light source and said lens, and a convex boundary surface opposite said light source having a center located on a straight line through said light source and center of said lens, said convex boundary surface substantially satisfying the condition R1 v/(1+v) and R2 (1+ 1/n) R1+t wherein: R1 is the radius of curvature in mm. of the concave boundary surface, v is the relative refraction index of the material of the lens for the wavelength range of the light from the light source R2 is the radius of curvature in mm. of the convex boundary surface and t is the thickness in mm. of the central part of the lens.
 4. An exposure device as claimed in claim 1 further comprising a corrective lens between said lens and mask for adapting the direction of said light radiated from said light source to the direction of deflected electron beams, said corrective lens having a filter layer, the permeability of said filter layer increasing from its center towards its edges. 